1. Field of the Invention
The present invention relates to a manipulator arm and drive system that can be operated in multiple modes, including an on or off mode, referred to herein as a “rate mode” or a spatially correspondent (“SC”) mode. The multi-mode manipulator arm and drive system of the present invention can be hydraulically operated subsea.
2. Description of the Prior Art
Prior art manipulator arms are available in two alternate primary modes (types), rate mode and spatially correspondent (“SC”) mode manipulators. In rate mode, each of the manipulator degrees-of-freedom (DOF) is controlled by an actuator which in turn is controlled via a directional control valve that is either fully on or fully off. While the term “rate mode” is familiar to those skilled in the manipulator arm art, it does not provide a literal description of the functional capabilities of this mode. In prior art rate mode, the manipulator joint is either moving at full speed or it is completely stopped. In prior art rate mode, the rate of movement of the manipulator arm is not controlled. It would be advantageous to have a single manipulator that could be selectively operated in either of these modes.
In rate mode operation, the rate mode controller allows simple, on/off control of one or more actuator control channels. This further causes actuation of the appropriate actuator which, in turn causes movement of the appropriate arm joint or segment. The operator may actuate more than one actuator at a time. The operator is not in control of the velocity of the joint or segment since it is simply an “on/off” function. In rate mode, joint position feedback is not present. The operator simply actuates the desired joint or segment until he sees that it is the desired position/orientation.
A rate mode manipulator arm and drive system suitable for subsea applications is shown in FIG. 1. The rate mode manipulator shown in FIG. 1 is suitable for use with a manipulator having a single degree of freedom. In rate mode, the operator energizes a directional control valve by depressing individual buttons or button in order to move the directional control valve, and hence the actuator, in the desired direction. Rate mode manipulators operate in an “open-loop” fashion wherein the operator depresses the corresponding button or buttons until the manipulator joint or joints move into the desired position. The operator monitors the position of the manipulator visually. In subsea applications using an ROV, this may be accomplished via a subsea camera. There is no position feedback signal utilized in the manipulator control electronics itself.
Rate mode provides a more awkward method of controlling a manipulator arm than SC mode; however, rate mode manipulation is simpler and less costly to implement than SC mode manipulation. A rate mode manipulator is also more reliable than an SC mode manipulator because it requires less electronics than an SC mode manipulator.
In the SC mode (also known as “position controlled mode”), the position of each manipulator arm joint is known and controlled. Typically, an SC manipulator system comprises two parts: a master and a slave. The master is an input device, often embodied in a hand controller that is equipped with a number of joints whose angular position is measured and monitored as the operator moves the controller. Generally, the master has a joint arrangement that mimics the joint arrangement of the slave.
The slave is the manipulator itself. The manipulator is a tele-robotic arm. The slave will move in proportion to the master hand controller. If a joint on the master is moved slowly, the slave joint will move slowly. If the master is moved quickly, the slave will move quickly. The movement (velocity) of the slave joints and segments “correspond” to the movement of the “master” controller joints and segments. An SC mode manipulator arm and drive system is shown in FIG. 2.
Positions or changes-in-position of the master's joints and segments is monitored by a local control computer. The local control computer sends the appropriate signals to the remote control computer in response to master controller inputs. The remote control computer monitors the position of the arm joints and segments and compares those positions with the position information sent from the local control computer. It then performs the necessary calculations to determine the direction and magnitude of the signals required from the actuator control in order to move the actuators, and hence the arm joints and segments, to the right position.
Prior art SC manipulators operate in “closed-loop” mode, which uses an error signal that represents the position of each and every joint on the slave. This signal is continuously compared to the desired joint position (as indicated by the position of the master's matching joint) and the direction and magnitude of the corresponding control valve is modulated as necessary according to some sort of algorithm which is usually a variant of a proportional, integral, derivative (PID) loop.
In existing manipulator or robotic arm designs, the angular displacement of one or more joints is monitored with a resolver, potentiometer, or other rotation sensor. These require some sort of mechanical connection, typically a shaft, between the moveable portion of the joint and the sensor. Sensors are typically held stationary by the non-moveable portion of the joint. In a subsea environment, mechanical connection, e.g. a shaft, must be equipped with a mechanical connection seal to prevent seawater intrusion into the sensor. This mechanical connection seal is prone to failure, thus resulting in the subsequent failure of the sensor.
Existing solutions require discrete wiring for each sensor installed. Arms with large numbers of joint sensors require considerable wiring that can be difficult to install and maintain.
Existing sensor types often require that some sort of host controller read analog values that are produced by the sensor, e.g. a resolver or potentiometer. This requires that the controller provide processing power to read, filter, and scale the readings of each of the sensors which have had to transmit analog signals over long, noise-prone conductors.
Prior art SC mode manipulator systems have several problems. Each joint of the slave must be equipped with a position feedback device such as an encoder, resolver, or potentiometer. The control algorithm must have a reliable signal from this device in order for the manipulator to work. If any of the feedback devices fail, then the manipulator is unusable.
The velocity and acceleration of the slave joints must be variable and, preferably, stepless. Traditionally, this has been achieved by using hydraulic servo valves which suffer four disadvantages, which are high cost, propensity for failure due to lack of fluid cleanliness, high leakage rate, and high pressure drop at high flow rates. In order to increase the longevity of the SC manipulator, an isolated hydraulic power unit (HPU) is often required. This adds to the cost, weight and complexity of the system.
SC mode manipulators are easier than rate mode manipulators to operate. They also provide the operator with a fluid touch. An SC mode manipulator requires more responsive valves and electronics than a rate mode manipulator. This results in increased complexity and reduced reliability for an SC mode manipulator versus a rate mode manipulator.